1. Field
This disclosure relates generally to integrated circuit memories, and more specifically, to phase change memory cells.
2. Related Art
A relatively new type of memory known as a phase change memory has been introduced which offers some advantage over existing non-volatile memories (NVMs). The phase change memory operates on the principle that there are phase change materials (PCMs) that change resistance upon a phase change and this change in resistance is reversible. One such material is a combination of germanium, antimony, and tellurium and is known as GST. The PCM is heated for a relatively short time and quickly cooled to achieve an amorphous state, which is high resistance. The PCM is heated at a lower temperature but for a longer time to achieve a crystalline state, which is low resistance. The difference in resistance is detectible and thus useful for defining two different logic states. One of the difficulties, however, is obtaining enough heat in order to achieve the amorphous state. Typically, heaters are metal that are either over and under the PCM or where the metal makes contact to the PCM in two locations on the top side. In the case of over and under, there are then multiple levels of vias required just to contact the PCM. In the case of both vias on the top side, the PCM must be big enough to have two contacts made to it. Also the contacts are preferably tapered to increase the resistance and thus the heat. Resistance is preferably not too low because the current is limited by transistors so that the heat is directly related to the resistance over an operable range.
Another issue with this type of memory is that contact between the PCM and a transistor must be made. The PCM must also make contact to a reference or a bit line. In either case that means that vias are required for two locations on a PCM.
Thus, there is a need for improving upon the issues pointed out above.